Stacked substrate structure with inter-tier interconnection

ABSTRACT

The present disclosure, in some embodiments, relates to a multi-dimensional integrated chip structure. The structure includes a first interconnect layer within a first dielectric structure on a first substrate, and a second interconnect layer within a second dielectric structure on a second substrate. A bonding structure is between the first dielectric structure and the second substrate. An inter-tier interconnect structure extends through the second substrate and between a top of the first interconnect layer and a bottom of the second interconnect layer. The inter-tier interconnect structure includes a first region having substantially vertical sidewalls extending through the second substrate and a second region below the first region and having tapered sidewalls surrounded by the bonding structure.

REFERENCE TO RELATED APPLICATIONS

This Application is a Continuation of U.S. application Ser. No.15/365,064, filed on Nov. 30, 2016, which claims priority to U.S.Provisional Application No. 62/272,128, filed on Dec. 29, 2015. Thecontents of the above-referenced Patent Applications are herebyincorporated by reference in their entirety.

BACKGROUND

A multi-dimensional integrated chip is an integrated circuit havingmultiple substrates or die which are vertically stacked onto andelectrically interconnected to one another. By electricallyinterconnecting the stacked substrates or die, the multi-dimensionalintegrated chip acts as a single device, which provides improvedperformance, reduced power consumption, and a smaller footprint overconvention integrated chips.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 illustrates a cross-sectional view of some embodiments of amulti-dimensional integrated chip structure having first and secondtiers connected in a front-to-back configuration.

FIG. 2 illustrates a cross-sectional view of some embodiments of afront-side illuminated CMOS image sensor (FSI-CIS) arranged within athree-dimensional integrated chip (3DIC) structure having tiersconnected in a front-to-back configuration.

FIG. 3 illustrates a cross-sectional view of some additional embodimentsof a FSI-CIS arranged within a 3DIC structure having tiers connected ina front-to-back configuration.

FIG. 4 illustrates a cross-sectional view of some additional embodimentsof a FSI-CIS arranged within a 3DIC structure having tiers connected ina front-to-back configuration.

FIGS. 5-14 illustrate cross-sectional views corresponding to someembodiments of methods of forming a multi-dimensional integrated chipstructure having tiers connected in a front-to-back configuration.

FIG. 15 illustrates a flow diagram of some embodiments of a method offorming a multi-dimensional integrated chip structure having tiersconnected in a front-to-back configuration.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

Integrated circuits (IC) with image sensors are used in a wide range ofmodern day electronic devices, such as cell phones and computers, forexample. Complementary metal-oxide semiconductor (CMOS) image sensors(CIS) have become popular types of IC image sensors. Compared tocharge-coupled devices (CCD), CMOS image sensors have low powerconsumption, small size, fast data processing, a direct output of data,and low manufacturing cost. Some types of CMOS image sensors includefront-side illuminated CMOS image sensors (FSI-CIS) and back-sideilluminated CMOS image sensors (BSI-CIS). FSI-CIS have image sensingelements arranged along a front-side of a substrate so that a pluralityof metal interconnect layers are arranged within an optical path of animage sensing element. BSI-CIS are arranged along a back-side of asubstrate and do not have metal interconnect layers within an opticalpath of an image sensing element.

CIS are increasingly being integrated into multi-dimensional integratedchip structures having a substrate with image sensing elements stackedonto a substrate with logic devices. The integration of CIS intomulti-dimensional integrated chip structures improves performance anddecreases power consumption over conventional packaging schemes.Typically, stacked integrated chip structures adopt a front-side tofront-side bonding that couples together back-end-of-the-line (BEOL)metallization stacks of logic and image sensor substrates. For BSI-CIS,the image sensing substrate is subsequently thinned to allow light topass through a back-side of the image sensing substrate. However, such astacked integrated chip structure is not suitable for FSI-CIS since itdoes not allow FSI-CIS to receive light.

The present disclosure relates to a method of forming amulti-dimensional integrated chip having tiers connected in afront-to-back configuration, and an associated apparatus. In someembodiments, the method is performed by forming one or moresemiconductor devices within a first substrate, and forming one or moreimage sensing elements within a second substrate. A first dielectricstructure over the first substrate is bonded to a back-side of thesecond substrate by way of a bonding structure. An inter-tierinterconnect structure, comprising a plurality of different segments,respectively having sidewalls with different sidewall angles, is formedto extend through the bonding structure and the second substrate. Theinter-tier interconnect structure is configured to electrically couple afirst metal interconnect layer over the first substrate to a secondmetal interconnect layer over the second substrate. Bonding the firstdielectric structure to a back-side of the second substrate allows forthe image sensing elements to be integrated as front-side image sensorswithin the multi-dimensional integrated chip structure.

FIG. 1 illustrates a cross-sectional view of some embodiments of amulti-dimensional integrated chip structure 100 having a first tierconnected to a second tier in a front-to-back configuration.

The multi-dimensional integrated chip structure 100 comprises a firsttier 102 and a second tier 114. The first tier 102 has a front-side 102fand a back-side 102 b. The second tier 114 also has a front-side 114 fand a back-side 114 b. The front-side 102f of the first tier 102 isbonded to the back-side 114 b of the second tier 114 by way of a bondingstructure 112 arranged between the first tier 102 and the second tier114. In some embodiments, the bonding structure 112 may comprise twodielectric (e.g., oxide) bonding layers abutting along a bondinginterface, for example.

The first tier 102 comprises a first substrate 104 arranged along theback-side 102 b and a first dielectric structure 108 arranged over thefirst substrate 104 along the front-side 102f. One or more semiconductordevices 106 are arranged within the first substrate 104. In variousembodiments, the one or more semiconductor devices 106 may comprisetransistor devices and/or passive devices, for example. A firstplurality of metal interconnect layers 110 are arranged within the firstdielectric structure 108.

The second tier 114 comprises a second substrate 116 arranged along theback-side 114 b and a second dielectric structure 120 arranged over thesecond substrate 116 along the front-side 114 f. In some embodiments,one or more image sensing elements 118, which are configured to generatecharge carriers (e.g., electron-hole pair) from incident radiation, arearranged within the second substrate 116. In other embodiments, thesecond substrate 116 may alternatively and/or additionally compriselogic devices, passive devices, MEMs devices, etc. A second plurality ofmetal interconnect layers 122 are arranged within the second dielectricstructure 120. In some embodiments, the second plurality of metalinterconnect layers 122 are arranged to have openings 126 that overliethe one or more image sensing elements 118. The openings 126 allowradiation incident on the front-side 114 f to reach the one or moreimage sensing elements 118.

An inter-tier interconnect structure 124 is configured to electricallycouple the first tier 102 and the second tier 114. The inter-tierinterconnect structure 124 extends from one of the first plurality ofmetal interconnect layers 110, through the bonding structure 112 and thesecond substrate 116, to one of the second plurality of metalinterconnect layers 122. In some embodiments, a diffusion barrier layer128 is arranged along sides of the inter-tier interconnect structure124. The inter-tier interconnect structure 124 may also be separatedfrom the second substrate 116 by way of an electrically isolating layer130 (e.g., an oxide).

The inter-tier interconnect structure 124 comprises a plurality ofdifferent segments, 124 a and 124 b, which respectively have sidewallswith different slopes (i.e., different sidewall angles). The pluralityof different segments, 124 a and 124 b, cause the inter-tierinterconnect structure 124 to have stepped sides, extending between topand bottom surfaces of the inter-tier interconnect structure 124, whichdo not extend along a straight line between the top and bottom surfaces.In some embodiments, the slopes of connected segment sidewalls may havedifferent polarities (e.g., a first segment may have a sidewall with apositive slope coupled to a second segment having a sidewall with anegative slope).

In some embodiments, the first substrate 104 may have a differentthickness than the second substrate 116. For example, in someembodiments, the first substrate 104 has a first thickness t₁ and thesecond substrate 116 has a second thickness t₂ that is less than thefirst thickness t₁. For example, in some embodiments, the firstthickness t₁ may be in a first range of between approximately 100 um andapproximately 1,000 um, while the second thickness t₂ may be in a secondrange of between approximately 1.5 um and approximately 100 um. Thesmaller second thickness t₂ of the second substrate 116 improvesperformance of the multi-dimensional integrated chip structure 100 bydecreasing interconnect distance between the first tier 102 and thesecond tier 114. It also allows for the inter-tier interconnectstructure 124 to be easily formed to couple the first tier 102 to thesecond tier 114 using multiple different etching processes. Thedifferent slopes of the inter-tier interconnect structure 124 are aresult of different etching processes that are used to form theinter-tier interconnect structure 124.

FIG. 2 illustrates a cross-sectional view of some embodiments of afront-side illuminated CMOS image sensor (FSI-CIS) within athree-dimensional integrated chip (3DIC) structure 200 having tiersconnected in a front-to-back configuration.

The 3DIC structure 200 comprises a first tier 102 having one or moresemiconductor devices 106 a-106 b arranged within a first substrate 104.In some embodiments, isolation structures 107 may separate adjacent onesof the one or more semiconductor devices 106 a-106 b. A first dielectricstructure 202 is arranged over the first substrate 104. In someembodiments, the first dielectric structure 202 comprises one or moreinter-level dielectric (ILD) layers 202 a-202 c that are verticallyseparated by etch stop layers 204. A first plurality of metalinterconnect layers 206 a-206 c are arranged within the first dielectricstructure 202. In some embodiments, the first plurality of metalinterconnect layers 206 a-206 c may comprise conductive contacts 206 a(CO), metal vias 206 b (Vx, where x=1:n), and metal interconnect wires206 c (Mx, where x=1:n). The conductive contacts 206 a electricallycouple the semiconductor devices 106 a-106 b to the metal interconnectwires 206 c, which are separated by the metal vias 206 b. In someembodiments, the first plurality of metal interconnect layers 206 a-206c may be separated from the first dielectric structure 108 by adiffusion barrier layer 208.

A second tier 114 is arranged over the first tier 102. The second tier114 comprises one or more pixel regions 209 respectively comprising animage sensing element arranged within a second substrate 116. In someembodiments, an array of pixel regions 209 may be arranged in rows andcolumns within the second substrate 116. In some embodiments, isolationstructures 216 (e.g., STI regions) are arranged on opposing sides of thepixel region 209, so as to provide for electrical isolation betweenadjacent pixel regions. The image sensing element may comprise aphotodiode 210 having a first region 210 a with a first doping type(e.g., n-type doping) abutting a second region 210 b with a seconddoping type (e.g., p-type doping) that is different than the firstdoping type. In some embodiments, the second substrate 116 may have thesecond doping type. In some embodiments, the first region 210 a and thesecond region 210 b may have doping concentrations greater than or equalto approximately 5e15 atoms/cm³.

In some embodiments, the photodiode 210 is operably coupled to a dopedregion 212 by way of a transistor gate 214 arranged over the secondsubstrate 116. The transistor gate 214 comprises a gate dielectric layerdisposed over the second substrate 116 and a gate electrode arrangedonto the gate dielectric layer. The transistor gate 214 is laterallyarranged between the photodiode 210 and the doped region 212 and isconfigured to convert light to an electrical signal that is sent to thefirst tier 102 by controlling the flow of the charge carriers from thephotodiode 210 to the doped region 212.

A second dielectric structure 218 is arranged over the second substrate116. In some embodiments, the second dielectric structure 218 comprisesone or more ILD layers 218 a-218 c that are separated by etch stoplayers 204. A second plurality of metal interconnect layers 220 a-220 care arranged within the second dielectric structure 218. In someembodiments, the second plurality of metal interconnect layers 220 a-220c may comprise conductive contacts 220 a, metal vias 220 b, and metalinterconnect wires 220 c. In various embodiments, the first plurality ofmetal interconnect layers 206 a-206 c and the second plurality of metalinterconnect layers 220 a-220 c may comprise copper, aluminum, and/ortungsten, and the diffusion barrier layer 208 may comprise tantalum,tantalum-nitride, titanium, and/or titanium-nitride. In variousembodiments, the one or more ILD layers, 202 a-202 c and 218 a-218 c,may comprise an oxide, an ultra-low k dielectric material, and/or alow-k dielectric material (e.g., SiCO), and the etch stop layers 204 maycomprise a nitride (e.g., silicon nitride).

In some embodiments, one or more passivation layers 222 are arrangedover the second dielectric structure 218. The one or more passivationlayers 222 may comprise oxide and/or nitride layers. A plurality ofcolor filters 236 are arranged over the one or more passivation layers222. The plurality of color filters 236 are respectively configured totransmit specific wavelengths of radiation. For example, a first colorfilter (e.g., a red color filter) may transmit radiation havingwavelengths within a first range, while a second color filter (e.g., agreen color filter) may transmit radiation having wavelengths within asecond range different than the first range. In some embodiments, theplurality of color filters 236 may be surrounded by a grid structure224. In some embodiments, the grid structure 224 may comprise a stackedstructure having a dielectric material 224 a (e.g., silicon nitride) andan overlying metal layer 224 b. The grid structure 224 forms a frameworkthat defines a plurality of openings located over underlying pixelregions 209.

A plurality of micro-lenses 238 are arranged over the plurality of colorfilters 236. The plurality of micro-lenses 238 are respectively alignedwith the color filters 236. In some embodiments, the plurality ofmicro-lenses 238 have a substantially flat bottom surface abutting thecolor filters 236, and a curved upper surface. The curved upper surfaceis configured to focus the incident radiation onto a center of anunderlying photodiode 210 to increase efficiency of the photodiode 210.In some embodiments, a bond pad (not shown) may also be arranged overthe one or more passivation layers 222 at a location laterally offsetfrom the plurality of micro-lenses 238. The bond pad is configured toextend through the one or more passivation layers 222 to contact one ofthe second plurality of metal interconnect layers 220 a-220 c.

The first dielectric structure 202 is bonded to a back-side 116 b of thesecond substrate 116 by way of a bonding structure 226 comprising afirst dielectric bonding layer 226 a and second dielectric bonding layer226 b. In some embodiments, the first dielectric bonding layer 226 a andthe second dielectric bonding layer 226 b may comprise oxide layers. Insome embodiments, a bonding etch stop layer (ESL) 228 is arrangedbetween the bonding structure 226 and the first dielectric structure202. In some embodiments, the bonding etch stop layer ESL 228 maycomprise a nitride (e.g., silicon nitride).

An inter-tier interconnect structure 230 extends from one of the firstplurality of metal interconnect layers 206 a-206 c to one of the secondplurality of metal interconnect layers 220 a-220 c. In variousembodiments, the inter-tier interconnect structure 230 may extend fromany one of the first plurality of metal interconnect layers 206 a-206 c(e.g., CO, Vx, or Mx) to any one of the second plurality of metalinterconnect layers 220 a-220 c. In some embodiments, the inter-tierinterconnect structure 230 may extend through an isolation structure 216(e.g., an STI region) within the second substrate 116. In suchembodiments, the isolation structure 216 provides for improved isolationbetween the pixel region 209 and the inter-tier interconnect structure230. In various embodiments, the inter-tier interconnect structure 230may comprise a conductive material, such as copper, tungsten, and/oraluminum, for example.

The inter-tier interconnect structure 230 is configured to electricallycouple the first tier 102 and the second tier 114. In some embodiments,by connecting the first tier 102 to the second tier 114 using theinter-tier interconnect structure 230, signal processing of signalsgenerated in the pixel regions 209 may be exclusively performed by thesemiconductor devices 106 a-106 b within the first tier 102.

The inter-tier interconnect structure 230 has a first segment 230 a anda second segment 230 b. The first segment 230 a is arranged within thebonding structure 226 and has tapered sidewalls that give the firstsegment 230 a a width that increases as a distance from the firstsubstrate 104 increases. In some embodiments, the first segment 230 amay extend from one of the first plurality of metal interconnect layers206 a-206 c to an upper surface of the bonding structure 226. The secondsegment 230 b has substantially vertical sidewalls that give the secondsegment 230 b a substantially constant width. In some embodiments, thesecond segment 230 b may extend from the back-side 116 b of the secondsubstrate 116 to one of the second plurality of metal interconnectlayers 220 a-220 c.

A diffusion barrier layer 232 extends along sidewalls of the firstsegment 230 a and the second segment 230 b. In various embodiments, thediffusion barrier layer 232 may comprise titanium (Ti), titanium nitride(TiN), tantalum (Ta), and/or tantalum nitride (TaN). The diffusionbarrier layer 232 has a lateral segment 231 extending along a lowersurface of the second segment 230 b. An isolation layer 234 extendsalong the diffusion barrier layer 232 from the back-side 116 b of thesecond substrate 116 to one of the second plurality of metalinterconnect layers 220 a-220 c. In some embodiments, the isolationlayer 234 comprises an oxide or a nitride, for example.

FIG. 3 illustrates a cross-sectional view of some alternativeembodiments of a FSI-CIS within a 3DIC structure 300 having tiersconnected in a front-to-back configuration.

The 3DIC structure 300 comprises an inter-tier interconnect structure302 having a first segment 302 a, a second segment 302 b, and a thirdsegment 302 c, which respectively comprise a conductive material. Thefirst segment 302 a has substantially vertical sidewalls that give thefirst segment 302 a a substantially constant width as a distance fromthe first substrate 104 increases. In some embodiments, the firstsegment 302 a may extend from a back-side 116 b of the second substrate116 to one of the second plurality of metal interconnect layers 220a-220 c. The second segment 302 b is arranged within a second dielectricbonding layer 226 b. The second segment 302 b extends between upper andlower surfaces of the second dielectric bonding layer 226 b. In someembodiments, the second segment 302 b may extend into a recess withinthe first segment 302 a. The second segment 302 b has tapered sidewallsthat give the second segment 302 b a width that decreases as a distancefrom the first substrate 104 increases. In some embodiments, the firstsegment 302 a may have a greater width than the second segment 302 b.The third segment 302 c is arranged within a first dielectric bondinglayer 226 a of the bonding structure 226. In some embodiments, the thirdsegment 302 c extends between an upper surface of the first dielectricbonding layer 226 a and one of the first plurality of metal interconnectlayers 206 a-206 c. The third segment 302 c has tapered sidewalls thatgive the first third 302 c a width that increases as a distance from thefirst substrate 104 increases.

A diffusion barrier layer 304 extends along sidewalls and a lowersurface of the third segment 302 c, along sidewalls and an upper surfaceof the second segment 302 b, and along sidewalls and a lower surface ofthe first segment 302 a. The diffusion barrier layer 304 separates thefirst segment 302 a from the second segment 302 b, while the secondsegment 302 b directly contacts the third segment 302 c. An isolationlayer 306 extends along the diffusion barrier layer 304 from theback-side 116 b of the second substrate 116 to one of the secondplurality of metal interconnect layers 220 a-220 c.

FIG. 4 illustrates a cross-sectional view of some alternativeembodiments of a FSI-CIS within a 3DIC structure 400 having tiersconnected in a front-to-back configuration.

The 3DIC structure 400 comprises a first tier 102 connected to a secondtier 114 by way of a bonding structure 226. The bonding structure 226 isseparated from the first tier 102 by way of a first bonding ESL 228 andis further separated from the second tier 114 by way of a second bondingESL 408.

An inter-tier interconnect structure 402 couples the first tier 102 tothe second tier 114. The inter-tier interconnect structure 402 comprisesa first segment 402 a, a second segment 402 b, a third segment 402 c,and a fourth segment 402 d, which respectively comprise a conductivematerial. The first segment 402 a is arranged within a second dielectricstructure 218 over a second substrate 116. In some embodiments, thefirst segment 402 a may extend from a front-side 116 f of the secondsubstrate 116 to one of a second plurality of metal interconnect layers220 a-220 c. The first segment 402 a has tapered sidewalls that give thefirst segment 402 a a width that decreases as a distance from the secondsubstrate 116 increases. In some embodiments, the second segment 402 bmay extend between a back-side 116 b and the front-side 116 f of thesecond substrate 116. The second segment 402 b has substantiallyvertical sidewalls that give the second segment 402 b a substantiallyconstant width as a distance from the first substrate 104 increases. Thethird segment 402 c is arranged within a second dielectric bonding layer226 b. In some embodiments, the third segment 402 c may extend betweenupper and lower surfaces of the second dielectric bonding layer 226 b.The third segment 402 c has tapered sidewalls that give the thirdsegment 402 c a width that decreases as a distance from the firstsubstrate 104 increases. The fourth segment 402 d is arranged within afirst dielectric bonding layer 226 a. In some embodiments, the fourthsegment 402 d extends between an upper surface of the first dielectricbonding layer 226 a and one of the first plurality of metal interconnectlayers 206 a-206 c. The fourth segment 402 d has tapered sidewalls thatgive the fourth segment 402 d a width that increases as a distance fromthe first substrate 104 increases.

A diffusion barrier layer 404 extends along sidewalls and a lowersurface of the fourth segment 402 d, along sidewalls and an uppersurface of the third segment 402 c, along sidewalls and a lower surfaceof the second segment 402 b, and along sidewalls and an upper surface ofthe first segment 402 a. The diffusion barrier layer 404 separates thesecond segment 402 b from the third segment 402 c, while the first andsecond segments, 402 a and 402 b, and the third and fourth segments, 402c and 402 d, directly contact one another. An isolation layer 406extends along the diffusion barrier layer 404 from the back-side 116 bto the front-side 116 f of the second substrate 116.

FIGS. 5-14 illustrate cross-sectional views 500-1400 corresponding tosome embodiments of methods of forming a multi-dimensional integratedchip structure having tiers connected in a front-to-back configuration.It will be appreciated that elements in FIGS. 5-14 that have beendescribed in previous embodiments have been designated with the samereference numbers for ease of understanding.

As shown in cross-sectional view 500 of FIG. 5, one or moresemiconductor devices 106 a-106 b are formed within a first substrate104. The first substrate 104 may be any type of semiconductor body(e.g., silicon, SiGe, SOI) such as a semiconductor wafer and/or one ormore die on a wafer, as well as any other type of semiconductor and/orepitaxial layers associated therewith. In some embodiments, the one ormore semiconductor devices 106 a-106 b may comprise transistor deviceshaving source/drain regions 502 separated by a channel region 504. Insuch embodiments, a gate structure 506 is formed over the channel region504. In some embodiments, isolation structures 107 (e.g., STI regions)are formed between adjacent semiconductor devices.

As shown in cross-sectional view 600 of FIG. 6, a first plurality ofmetal interconnect layers 206 a-206 c are formed within a firstdielectric structure 202 formed over the first substrate 104. In someembodiments, the first plurality of metal interconnect layers 206 a-206c may be formed by a damascene process and/or a dual damascene process.In such embodiments, a plurality of ILD layers 202 a-202 c are formedover the first substrate 104. The ILD layers 202 a-202 c are separatelyetched to form via holes and/or metal trenches. The via holes and/ormetal trenches are then filled with a conductive material to form one ormore of the first plurality of metal interconnect layers 206 a-206 c. Insome embodiments, the ILD layers 202 a-202 c may be deposited by aphysical vapor deposition technique (e.g., PVD, CVD, PE-CVD, ALD, etc.).The first plurality of metal interconnect layers 206 a-206 c may beformed using a deposition process and/or a plating process (e.g.,electroplating, electro-less plating, etc.).

A first bonding etch stop layer (ESL) 602 is formed onto the firstdielectric structure 202. In some embodiments, the first bonding ESL 602may comprise a nitride layer. A first dielectric bonding layer 604 isformed onto the first bonding ESL 602. In some embodiments, the firstbonding ESL 602 and the first dielectric bonding layer 604 may be formedby deposition processes (e.g., CVD, PVD, PE-CVD, ALD, etc.).

As shown in cross-sectional view 700 of FIG. 7, a second substrate 702is provided. In some embodiments, a plurality of image sensing elementsare formed within a pixel region 209 of the second substrate 702. Inother embodiments, one or more of a transistor device, a passive device,and/or a MEMs device may alternatively or additionally be formed withinthe second substrate 702.

In some embodiments, the plurality of image sensing elements mayrespectively comprise a photodiode 210 having abutting regions withdifferent doping types. In such embodiments, the photodiode 210 may beformed by selectively implanting the second substrate 702 with a firstimplantation process performed according to a first masking layer and asecond subsequent implantation process performed according to a secondmasking layer. In some embodiments, a doped region 212 may also beformed within the pixel region 209 by selectively implanting the secondsubstrate 702 with one or more dopant species.

A transistor gate 214 may be subsequently formed between the photodiode210 and the doped region 212. The transistor gate 214 may be formed bydepositing a gate dielectric film and a gate electrode film over thesecond substrate 702. The gate dielectric film and the gate electrodefilm are subsequently patterned to form a gate dielectric layer and agate electrode. In some embodiments, one or more isolation structures216 (e.g., shallow trench isolation regions) may be formed within thesecond substrate 702 on opposing sides of the pixel region 209. In someembodiments, the one or more isolation structures 216 may be formedprior to formation of the plurality of image sensing elements.

As shown in cross-sectional view 800 of FIG. 8, one or more metalinterconnect layers 220 a are formed within one or more dielectriclayers 802 overlying the second substrate 702. In some embodiments, oneor more metal interconnect layers 220 a may be formed using a damasceneand/or a dual damascene process.

As shown in cross-sectional view 900 of FIG. 9, after forming one ormore metal interconnect layers 220 a, the second substrate 702 isthinned. Thinning the second substrate 702 reduces a thickness of thesecond substrate 702 from a thickness t to form a second substrate 902having a thickness t₂. Reducing the thickness allows for the secondsubstrate 902 to be easily connected to the first substrate 104 using aninter-tier interconnect structure. In some embodiments, the secondsubstrate 702 may be thinned by etching a back-side 702 b of the secondsubstrate 702. In other embodiments, the second substrate 702 may bethinned by mechanically grinding the back-side 702 b of the secondsubstrate 702.

In some embodiments, the one or more dielectric layers 802 are bonded toa handle substrate 904 prior to thinning. In some embodiments, thebonding process may use an intermediate bonding oxide layer (not shown)arranged between the one or more dielectric layers 802 and the handlesubstrate 904. In some embodiments, the bonding process may comprise afusion bonding process. In some embodiments, the handle substrate 904may comprise a silicon wafer. After bonding the one or more dielectriclayers 802 to the handle substrate 904, the second substrate 702 may bethinned.

FIGS. 10A-10E illustrate cross-sectional views of some embodimentsshowing the formation of an inter-tier interconnect structure configuredto connect first and second tiers in a front-to-back configuration.

As shown in cross-sectional view 1000 of FIG. 10A, a second dielectricbonding layer 1002 is formed on a back-side 902 b of the secondsubstrate 902. The first dielectric bonding layer 604 is subsequentlybonded to the second dielectric bonding layer 1002 along adielectric-dielectric (e.g., oxide-oxide) bonding interface 1006. Thebonding process forms a bonding structure 1004 that connects the firstdielectric structure 202 to the second substrate 902. In someembodiments, the bonding process may comprise a fusion bonding process,for example. In some embodiments, the bonding process may comprise a‘wafer level bonding process’ that bonds the first substrate 104, whichcomprises a wafer (e.g., a first 300 mm wafer) to the second substrate116, which also comprises a wafer (e.g., a second 300 mm wafer).

As shown in cross-sectional view 1008 of FIG. 10B, a first etchingprocess is performed in a direction of a front-side of the secondsubstrate 116 using a first etchant 1010 to form a first opening 1012.The first opening 1012 extends through ILD layer 218 a and the secondsubstrate 116 to contact the bonding structure 1004. In someembodiments, the first etching process may comprise a first anisotropicetching process that results in substantially vertical sidewalls (e.g.,a dry anisotropic etching process). An isolation layer 234 is formed onsidewalls of the first opening 1012. In some embodiments, the isolationlayer 234 may comprise an oxide layer. In various embodiments, theisolation layer 234 may be formed by a deposition process or by athermal process.

As shown in cross-sectional view 1014 of FIG. 10C, a second etchingprocess is performed using a second etchant 1016 to from a secondopening 1018. The second opening 1018 extends through the bondingstructure 226 to contact one of the first plurality of metalinterconnect layers 206 a-206 c. In some embodiments, the second etchingprocess may comprise a second anisotropic etching process that resultsin tapered sidewalls (e.g., a wet anisotropic etching process).

As shown in cross-sectional view 1020 of FIG. 10D, a diffusion barrierlayer 232 is formed along sidewalls and bottom surfaces of the firstopening (e.g., 1012 of FIG. 10C) and the second opening (e.g., 1018 ofFIG. 10C). The first opening (e.g., 1012 of FIG. 10C) and the secondopening (e.g., 1018 of FIG. 10C) are subsequently filled with aconductive material to form a first segment 230 a and a second segment230 b of an inter-tier interconnect structure 230. A planarizationprocess (e.g., a CMP process) may be subsequently performed to removeexcess conductive material from outside of the first opening (e.g., 1012of FIG. 10C) and the second opening (e.g., 1018 of FIG. 10C).

As shown in cross-sectional view 1022 of FIG. 10E, one or more metalinterconnect layers, 220 b and 220 c, are formed within a dielectricstructure 218 having one or more ILD layers, 218 b and 218 c, overlyingILD layer 218 a. One of the one or more metal interconnect layers, 220 band 220 c, is formed onto the first segment 230 a. In some embodiments,one or more metal interconnect layers, 220 b and 220 c, may be formedusing a damascene and/or dual damascene process.

FIGS. 11A-11E illustrates cross-sectional views of some alternativeembodiments showing the formation of an inter-tier interconnectstructure configured to connect first and second tiers in afront-to-back configuration.

As shown in cross-sectional view 1100 of FIG. 11A, a second dielectricbonding layer 1102 is formed on a back-side 116 b of the secondsubstrate 116. A first etching process is subsequently performed in adirection of a front-side of the second substrate 116 using a firstetchant 1104 to from a first opening 1106. The first opening 1106extends through ILD layer 218 a and the second substrate 116 to contactthe second dielectric bonding layer 1102. In some embodiments, the firstetching process may comprise a first anisotropic etching process thatresults in substantially vertical sidewalls (e.g., a dry anisotropicetching process).

An isolation layer 306 is subsequently formed on sidewalls of the firstopening 1106. In some embodiments, the isolation layer 306 may comprisean oxide layer formed by a deposition or thermal process. A diffusionbarrier layer 1108 a is formed along sidewalls of the isolation layer306 and a bottom surface of the first opening 1106. The first opening1106 is subsequently filled with a first conductive material to form afirst segment 302 a of an inter-tier interconnect structure. Aplanarization process may be subsequently performed to remove excessconductive material from outside of the first opening 1106.

As shown in cross-sectional view 1110 of FIG. 11B, one or more metalinterconnect layers, 220 b and 220 c, are formed within a dielectricstructure having one or more ILD layers, 218 b and 218 c, overlying thesecond substrate 116. One of the one or more metal interconnect layers,220 b and 220 c, is formed onto the first segment 302 a. In someembodiments, one or more metal interconnect layers, 220 b and 220 c, maybe formed using a damascene and/or dual damascene process. In somealternative embodiments, the one or more metal interconnect layers, 220b and 220 c, may be formed after FIG. 11E.

As shown in cross-sectional view 1112 of FIG. 11C, a second etchingprocess is performed in a direction of a back-side of the secondsubstrate 116 using a second etchant 1114 to form a second opening 1116extending through the second dielectric bonding layer 226 b. In someembodiments, the second etching process may comprise an anisotropicetching process that results in the second opening 1116 having taperedsidewalls (e.g., a wet anisotropic etching process). A diffusion barrierlayer 1108 b is formed along sidewalls and bottom surface of the secondopening 1116. The second opening 1116 is subsequently filled with asecond conductive material to form a second segment 302 b of theinter-tier interconnect structure. A planarization process may besubsequently performed to remove excess conductive material from outsideof the second opening 1116.

As shown in cross-sectional view 1118 of FIG. 11D, a third etchingprocess is performed in a direction of a front-side of the firstsubstrate 104 using a third etchant 1120 to form a third opening 1122extending through the first dielectric bonding layer 226 a and the firstbonding ESL 228. In some embodiments, the third etching process maycomprise an anisotropic etching process that results in the thirdopening 1122 having tapered sidewalls (e.g., a wet anisotropic etchingprocess). A diffusion barrier layer 1108 c is formed along sidewalls andbottom surface of the third opening 1122. The third opening 1122 issubsequently filled with a third conductive material to form a thirdsegment 302 c of the inter-tier interconnect structure. In variousembodiments, the first, second and third conductive materials maycomprise a same material or different materials. A planarization processmay be subsequently performed to remove excess conductive material fromoutside of the third opening 1122.

As shown in cross-sectional view 1124 of FIG. 11E, the first dielectricbonding layer 226 a and the third segment 302 c of the inter-tierinterconnect structure are bonded to the second dielectric bonding layer226 b and the second segment 302 b of the inter-tier interconnectstructure along a hybrid bonding interface 1126 comprising a dielectricmaterial and a conductive material. In some embodiments, the bondingprocess is a ‘wafer level bonding process’ that bonds the firstsubstrate 104, which comprises a wafer (e.g., a first 300 mm wafer) to athe second substrate 116, which comprises a wafer (e.g., a second 300 mmwafer).

FIGS. 12A-12E illustrate cross-sectional views of some alternativeembodiments showing the formation of an inter-tier interconnectstructure configured to connect first and second tiers in afront-to-back configuration.

As shown in cross-sectional view 1200 of FIG. 12A, one or more metalinterconnect layers, 220 b and 220 c, are formed within a dielectricstructure having one or more ILD layers, 218 b and 218 c, overlying thesecond substrate 902. In some embodiments, one or more metalinterconnect layers, 220 b and 220 c, may be formed using a damasceneand/or dual damascene process.

As shown in cross-sectional view 1202 of FIG. 12B, a first etchingprocess is performed in a direction of the back-side of the secondsubstrate 116 to form a first opening 1204 extending through the secondsubstrate 116 and having substantially vertical sidewalls. An isolationlayer 406 is formed along sidewalls of the first opening 1204. A secondetching process is subsequently performed in a direction of theback-side of the second substrate 116 to form a second opening 1206extending through a first ILD layer 218 a to one of the metalinterconnect layers 220 a-220 c. The second opening 1206 has a widththat decreases as a distance from the second substrate 116 increases. Insome embodiments, the second opening 1206 may extend into a recesswithin one of second plurality of metal interconnect layers 220 a-220 c.A diffusion barrier layer 1208 a is formed along sidewalls of theisolation layer 406 and sidewalls and a lower surface of the secondopening 1206. The first opening 1204 and the second opening 1206 aresubsequently filled with a first conductive material to form a firstsegment 402 a and a second segment 402 b of an inter-tier interconnectstructure.

As shown in cross-sectional view 1210 of FIG. 12C, a second bonding ESL408 and a second dielectric bonding layer 226 b are formed on aback-side of the second substrate 116. A third etching process isperformed using a third etchant 1212 to from a third opening 1214extending through the second bonding ESL 408 and the second dielectricbonding layer 226 b. In some embodiments, the third etching process maycomprise an anisotropic etching process that results in the thirdopening 1214 having tapered sidewalls (e.g., a wet anisotropic etchingprocess). The third opening 1214 is subsequently filled with a diffusionbarrier layer 1208 b and a second conductive material to form a thirdsegment 402 c of the inter-tier interconnect structure.

As shown in cross-sectional view 1216 of FIG. 12D, a fourth etchingprocess is performed using a fourth etchant 1218 to form a fourthopening 1220 extending through the first dielectric bonding layer 226 aand the first bonding ESL 228. In some embodiments, the fourth etchingprocess may comprise an anisotropic etching process that results in thefourth opening 1220 having tapered sidewalls (e.g., a wet anisotropicetching process). A diffusion barrier layer 1208 c is formed alongsidewalls and bottom surface of the fourth opening 1220. The fourthopening 1220 is subsequently filled with a third conductive material toform a fourth segment 402 d of the inter-tier interconnect structure. Aplanarization process may be subsequently performed to remove excessconductive material from outside of the fourth opening 1220.

As shown in cross-sectional view 1222 of FIG. 12E, the first dielectricbonding layer 226 a and the fourth segment 402 d of the inter-tierinterconnect structure are bonded to the second dielectric bonding layer226 b and the third segment 402 c of the inter-tier interconnectstructure along a hybrid bonding interface region 1224 comprising adielectric material and a conductive material. In some embodiments, thebonding process is a ‘wafer level bonding process’ that bonds the firstsubstrate 104, which comprises a wafer (e.g., a first 300 mm wafer) to athe second substrate 116, which comprises a wafer (e.g., a first 300 mmwafer).

As shown in cross-sectional view 1300 of FIG. 13, a plurality of colorfilters 236 are formed over the second dielectric structure 218. In someembodiments, one or more passivation layers 222 may be formed over thesecond dielectric structure 218 prior to formation of the plurality ofcolor filters 236. The plurality of color filters 236 may be formedwithin a grid structure 224 overlying the second dielectric structure218. In some embodiments, the plurality of color filters 236 may beformed by forming a color filter layer and patterning the color filterlayer. The color filter layer is formed of a material that allows forthe transmission of radiation (e.g., light) having a specific range ofwavelength, while blocking light of wavelengths outside of the specifiedrange. The patterning may be performed by forming a photoresist layerwith a pattern over the color filter layer, applying an etchant to thecolor filter layer according to the pattern of the photoresist layer,and removing the pattern photoresist layer. In some embodiments, thecolor filter layer is planarized subsequent to formation.

A plurality of micro-lenses 238 are subsequently formed over theplurality of color filters 236. In some embodiments, the micro-lenses238 may be formed by depositing a micro-lens material above theplurality of color filters 236 (e.g., by a spin-on method or adeposition process). A micro-lens template (not shown) having a curvedupper surface is patterned above the micro-lens material. In someembodiments, the micro-lens template may comprise a photoresist materialexposed using a distributing exposing light dose (e.g., for a negativephotoresist more light is exposed at a bottom of the curvature and lesslight is exposed at a top of the curvature), developed and baked to forma rounding shape. The micro-lenses 238 are then formed by selectivelyetching the micro-lens material according to the micro-lens template.

In some embodiments, the plurality of color filters 236 and theplurality of micro-lenses 238 are formed over a stacked wafer structure1302, in which the first substrate 104 comprises a first wafer and thesecond substrate 116 comprises a second wafer (e.g., a thinned wafer).The stacked wafer structure 1302 comprises a plurality of die regions1304 a-1304 c. Each of the plurality of die regions 1304 a-1304 c maycomprise an image sensor array comprising a plurality of photodiodes 210arranged in a periodic pattern. In some embodiments, an image sensingarray within each of the plurality of die regions 1304 a-1304 c maycomprise hundreds, thousands, or even more individual photodiodes 210.For example, although die region 1304 b illustrates two photodiodes 210,it will be appreciated that the die region 1304 b may comprise an imagesensing array having more than two photodiodes. The die regions 1304a-1304 c also respectively comprise a plurality of color filters 236 andmicro-lenses 238 arranged in an array, such that a color filter 236 anda micro-lens 238 are positioned over an individual photodiode 210. Thearrays of color filters 236 and micro-lenses 238 are laterally set backfrom edges of a die region, 1304 a-1304 c. The lateral set back causesadjacent arrays of color filters 236 and micro-lenses 238 to belaterally separated from one another by a non-zero space 1306.

As shown in cross-sectional view 1400 of FIG. 14, in some embodiments,wherein the first substrate 104 comprises a first wafer (e.g., a first300 mm wafer) and the second substrate 116 comprises a second wafer(e.g., a second 300 mm wafer), a dicing process is performed. The dicingprocess separates the stacked wafer structure 1402 into a plurality ofseparate die, 1406 a-1406 c, respectively having multiple tiers, 102 and114. In some embodiments, the dicing process may be performed by way ofa sawing process (e.g., using a saw blade 1405) along a scribe line1404. In other embodiments, the dicing process may be performed by alaser along a scribe line 1404.

FIG. 15 illustrates a flow diagram of some embodiments of a method 1500of forming a multi-dimensional integrated chip structure having tiersconnected in a front-to-back configuration.

While the disclosed method 1500 is illustrated and described herein as aseries of acts or events, it will be appreciated that the illustratedordering of such acts or events are not to be interpreted in a limitingsense. For example, some acts may occur in different orders and/orconcurrently with other acts or events apart from those illustratedand/or described herein. In addition, not all illustrated acts may berequired to implement one or more aspects or embodiments of thedescription herein. Further, one or more of the acts depicted herein maybe carried out in one or more separate acts and/or phases.

At 1502, one or more semiconductor devices are formed within a firstsubstrate. FIG. 5 illustrates some embodiments of a cross-sectional view500 corresponding to act 1502.

At 1504, a first plurality of metal interconnect layers are formedwithin a first dielectric structure along a front-side of the firstsubstrate. FIG. 6 illustrates some embodiments of a cross-sectional view600 corresponding to act 1504.

At 1506, a first dielectric bonding layer is formed on a top surface ofthe first dielectric structure. FIG. 6 illustrates some embodiments of across-sectional view 600 corresponding to act 1506.

At 1508, one or more image sensing elements are formed within a secondsubstrate. FIG. 7 illustrates some embodiments of a cross-sectional view700 corresponding to act 1508.

At 1510, one or more second metal interconnect layers are formed withinone or more dielectric layers dielectric structure arranged along afront-side of the second substrate. FIG. 8 illustrates some embodimentsof a cross-sectional view 800 corresponding to act 1510.

At 1512, a thickness of the second substrate is reduced. FIG. 9illustrates some embodiments of a cross-sectional view 900 correspondingto act 1512.

At 1514, a second dielectric bonding layer is formed on a back-side ofthe second substrate.

At 1516, the first dielectric bonding layer is bonded to the seconddielectric bonding layer.

At 1518, an inter-tier interconnect structure is formed having aplurality of segments with sidewalls having different slopes. Theinter-tier interconnect structure is configured to electrically coupledthe first substrate to the second substrate.

At 1520, one or more additional metal interconnect layers may be formedover the front-side of second substrate, in some embodiments. FIGS.10A-10E illustrate some embodiments of cross-sectional viewscorresponding to acts 1514-1520. FIGS. 11A-11E illustrate somealternative embodiments of cross-sectional views corresponding to acts1514-1520. FIGS. 12A-12E illustrate some alternative embodiments ofcross-sectional views corresponding to acts 1514-1520.

At 1522, one or more color filters and one or more micro-lenses areformed over the image sensing elements within the second substrate. FIG.13 illustrates some embodiments of a cross-sectional view 1300corresponding to act 1522.

At 1524, a dicing process is performed to separate the first and secondsubstrates into a plurality of separate die, respectively havingmultiple tiers. FIG. 14 illustrates some embodiments of across-sectional view 1400 corresponding to act 1524.

Therefore, the present disclosure relates to a method of forming amulti-dimensional integrated chip having tiers connected in afront-to-back configuration, and an associated apparatus.

In some embodiments, the present disclosure relates to a method offorming a multi-dimensional integrated chip structure. The methodcomprises forming one or more semiconductor devices within a firstsubstrate, and forming one or more image sensing elements within asecond substrate. The method further comprises bonding a firstdielectric structure over the first substrate to a back-side of thesecond substrate by way of a bonding structure. The method furthercomprises forming an inter-tier interconnect structure, comprising aplurality of different segments that respectively have sidewalls withdifferent sidewall angles, which extends through the bonding structureand the second substrate. The inter-tier interconnect structure isconfigured to electrically couple a first metal interconnect layer overthe first substrate to a second metal interconnect layer over the secondsubstrate.

In other embodiments, the present disclosure relates to a method offorming a multi-dimensional integrated chip structure. The methodcomprises forming one or more semiconductor devices within a firstsubstrate, forming a first plurality of metal interconnect layers withina first dielectric structure along a front-side of the first substrate,and forming a first dielectric bonding layer on a top surface of thefirst dielectric structure. The method further comprises forming one ormore image sensing elements within a second substrate, forming one ormore second metal interconnect layers within one or more dielectriclayers arranged along a front-side of the second substrate, and forminga second dielectric bonding layer along a back-side of the secondsubstrate. The method further comprises bonding the first dielectricbonding layer to the second dielectric bonding layer. The method furthercomprises forming an inter-tier interconnect structure, having aplurality of different segments with sidewalls having different slopes,which is configured to electrically couple the one or more semiconductordevices and the one or more image sensing elements.

In yet other embodiments, the present disclosure relates to amulti-dimensional integrated chip structure. The integrated chipstructure comprises a first plurality of metal interconnect layersarranged within a first dielectric structure over a first substrate, anda second plurality of metal interconnect layers arranged within a seconddielectric structure over a second substrate. A bonding structure isarranged between the first dielectric structure and the secondsubstrate. An inter-tier interconnect structure extends between one ofthe first plurality of metal interconnect layers and one of the secondplurality of metal interconnect layers and through the bonding structureand the second substrate. The inter-tier interconnect structurecomprises a plurality of different segments that respectively havesidewalls with different sidewall angles.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A multi-dimensional integrated chip structure,comprising: a first interconnect layer within a first dielectricstructure on a first substrate; a second interconnect layer within asecond dielectric structure on a second substrate; a bonding structurebetween the first dielectric structure and the second substrate; and aninter-tier interconnect structure extending through the second substrateand between a top of the first interconnect layer and a bottom of thesecond interconnect layer, wherein the inter-tier interconnect structurecomprises a first region having substantially vertical sidewallsextending through the second substrate and a second region below thefirst region and having tapered sidewalls surrounded by the bondingstructure.
 2. The structure of claim 1, further comprising: a dielectricmaterial lining sidewalls of the second substrate and the seconddielectric structure, wherein a bottom of the dielectric material is ona top of the bonding structure facing the second substrate.
 3. Thestructure of claim 2, wherein the inter-tier interconnect structurefurther comprises: a barrier layer arranged along sidewalls of the firstregion and protruding outward past the bottom of the dielectric materialtowards the first substrate.
 4. The structure of claim 1, wherein thetapered sidewalls extend from within the bonding structure to ahorizontal line extending along a bottom of the second substrate.
 5. Thestructure of claim 1, wherein the tapered sidewalls extend from a bottomof the bonding structure to a horizontal line extending along a bottomof the second substrate.
 6. The structure of claim 1, wherein theinter-tier interconnect structure further comprises: a first barrierlayer arranged along sidewalls of the first region; and a second barrierlayer arranged along sidewalls of the second region and directly betweena bottom of the first region and a top of the second region.
 7. Thestructure of claim 1, wherein the inter-tier interconnect structurefurther comprises: a third region between the first region and thesecond dielectric structure, wherein the third region has a width thatdecreases as a distance from the second substrate increases.
 8. Thestructure of claim 1, wherein the inter-tier interconnect structurefurther comprises: a third region between the second region and thefirst dielectric structure, wherein the third region has a width thatdecreases as a distance from the second substrate increases.
 9. Thestructure of claim 8, wherein the third region laterally extends pastone of the tapered sidewalls of the second region by a non-zerodistance.
 10. The structure of claim 8, further comprising: an etch stoplayer arranged between the bonding structure and the second substrate.11. The structure of claim 10, wherein the etch stop layer continuouslyextends from directly below the second substrate to directly below theinter-tier interconnect structure.
 12. A multi-dimensional integratedchip structure, comprising: a first plurality of interconnect layerswithin a first dielectric structure on a first substrate; a secondplurality of interconnect layers within a second dielectric structure ona second substrate; and an inter-tier interconnect structureelectrically connecting the first plurality of interconnect layers andthe second plurality of interconnect layers and extending through thesecond substrate, wherein the inter-tier interconnect structurecomprises a first sidewall extending through the second substrate andcoupled to a tapered second sidewall by a horizontally extending surfaceof the inter-tier interconnect structure.
 13. The structure of claim 12,further comprising: a dielectric bonding structure arranged between aback-side of the second substrate and the first dielectric structure,wherein the tapered second sidewall is between tapered sidewalls of thedielectric bonding structure.
 14. The structure of claim 13, furthercomprising: a plurality of gate structures arranged along a front-sideof the second substrate; and an etch stop layer arranged between thedielectric bonding structure and the back-side of the second substrate.15. An integrated chip structure, comprising: a first interconnect layerwithin a first dielectric structure on a first substrate; a secondinterconnect layer within a second dielectric structure on a secondsubstrate; a dielectric bonding structure arranged between the firstsubstrate and the second substrate; an inter-tier interconnect structureextending between the first interconnect layer and the secondinterconnect layer and through the dielectric bonding structure and thesecond substrate; and an isolation layer lining sidewalls of the secondsubstrate and the second dielectric structure, wherein a bottom of theisolation layer is on a top of the dielectric bonding structure facingthe second substrate.
 16. The structure of claim 15, wherein theinter-tier interconnect structure further comprises: a barrier layerarranged along sidewalls of a conductive material and protruding outwardpast the bottom of the isolation layer towards the first substrate. 17.The structure of claim 15, wherein the inter-tier interconnect structurehas tapered sidewalls that continuously extend from within thedielectric bonding structure to a horizontal line extending along abottom of the second substrate.
 18. The structure of claim 15, whereinthe inter-tier interconnect structure has a width that increases asdistances from opposing sides of the dielectric bonding structureincrease.
 19. The structure of claim 15, wherein the inter-tierinterconnect structure has tapered sidewalls directly between taperedsidewalls of the dielectric bonding structure.
 20. The structure ofclaim 19, wherein the inter-tier interconnect structure has additionalsidewalls arranged directly between sidewalls of the second substrate,the additional sidewalls are angled at a non-zero angle with respect tothe tapered sidewalls.